1. Field of the Invention
The present invention relates to a device for detecting knocking in an internal combustion engine. More particularly, the invention relates to a device for detecting knocking in an internal combustion engine which is capable of preventing the accuracy of detecting knocking from being deteriorated by noise when an LC resonance masking is reset.
2. Prior Art
In an internal combustion engine using gasoline as a fuel, the gas mixture compressed by a piston is ignited by a spark plug and is burned to produce an output. That is, in the normal combustion, a nucleus of a burning gas mixture is formed near the gap of the spark plug, and the flame propagates over the combustion chamber.
The ignition timing of the spark plug has an intimate relationship with the output of the internal combustion engine. When the ignition timing is too late, the propagation speed of the flame becomes slow. Therefore, the combustion becomes slow resulting in a decrease in the combustion efficiency and, hence, in a decrease in the output of the internal combustion engine.
When the ignition timing is early, on the other hand, the propagation of the flame is fast, whereby a maximum pressure of combustion rises and the output of the internal combustion engine increases. When the ignition timing is too early, however, there takes place knocking in which the gas mixture self-ignites prior to the propagation of flame, often damaging the internal combustion engine.
That is, it is advantageous to operate the internal combustion engine in a region in which the ignition timing is set to just before the occurrence of knocking (MBT: minimum spark advance for best torque) from the standpoint of fuel efficiency and output. It is very important to reliably detect the occurrence of knocking.
A knock sensor which is a vibration sensor has heretofore been used for detecting knocking. However, a device has been studied which detects knocking by utilizing the phenomenon that ions are generated in the combustion chamber due to the combustion of the gas mixture and an ionic current flows.
FIG. 1 is a diagram schematically illustrating an ignition circuit for an internal combustion engine, wherein an end of a primary coil 111 of an ignition coil 11 is connected to the positive electrode of a battery 12. The other end is grounded via the collector and the emitter of a switching transistor 13 included in an igniter.
The base of the transistor 13 is connected to an ignition timing control unit 14, so that the transistor 13 is turned on when an ignition signal IGT is output from the ignition timing control unit 14.
An end of a secondary coil 112 of the ignition coil 11 is also connected to the positive electrode of the battery 12, and the other end is connected to a spark plug 18 through a reverse-current preventing diode 15, a distributor 16 and a high-tension cable 17.
An ionic current detecting unit 19 is connected to the other end of the secondary coil 112 of the ignition coil 11 in parallel with the spark plug 18.
The ionic current is supplied, through a protection diode 191, to a series circuit of a current-to-voltage conversion resistor 192 and a bias power source 193. A voltage generated at a point where the current-to-voltage conversion resistor 192 and the protection diode 191 are connected together, is applied to an amplifying circuit 195 composed of an operational amplifier and a resistor through a capacitor 194 for removing a DC component.
Therefore, a voltage signal proportional to the AC component of the ionic current is output at an output terminal 196 of the ionic current detecting unit 19.
FIGS. 2A to 2E are diagrams of waveforms at each of the portions of the ignition circuit (FIG. 1) and show, respectively, an ignition signal IGT, a voltage on the grounding side of the primary coil (point P), a voltage on the high-tension side of the secondary coil (point S), and a voltage input to the amplifying circuit (point I). All abscissa represent time.
When the ignition signal IGT turns to the "H" level at t.sub.1, and the transistor 13 is turned on and the voltage at point P drops. Immediately after .sub.1, a negative high-voltage pulse is generated at point S, that is, on the high-voltage side of the secondary coil. However, the current is blocked by the reverse current-preventing diode 15 from flowing into the spark plug 18.
When the ignition signal IGT turns to the "L" level at t.sub.2 and the transistor 13 is cut off, a voltage at point P abruptly rises, and a positive high-voltage pulse is generated at point S.
The positive high-voltage pulse is not blocked by the reverse current-preventing diode 15 and flows into the spark plug 18 to be discharged. It is prevented by the protection diode 191 from flowing into the ionic current detecting unit 19.
Furthermore, from t.sub.3 to t.sub.4 after the discharge of the spark plug 18, LC resonance is triggered by energy remaining in the ignition coil 11 due to parastic inductance and parastic capacitance of the high-tension cable 17 and the like.
The gas mixture in the cylinder is ignited by the discharge of the spark plug 18, ions are generated in the cylinder as the flame spreads, and an ionic current starts flowing. The ionic current increases with an increase in the pressure in the cylinder and decreases with a decrease in the pressure in the cylinder.
When knocking occurs in the internal combustion engine, knocking signals of a particular frequency (about 6 KHz) are superposed while the ionic current decreases after having reached its peak.
In order to detect knocking using the ionic current, therefore, it is desired to detect only the knocking signals in the particular frequency and reject other signals (e.g., LC resonance wave). For this purpose, therefore, it is desired to provide a knocking signal window which opens at t.sub.5 after no extra signal exists and closes at a suitable moment (e.g., ATDC 60.degree.) after the ionic current has decreased, and to detect the knocking based upon the output of the ionic current detecting unit 19 while the knocking signal window is open.
The ionic current detecting unit 19, however, detects a very small ionic current and the amplifying circuit 195 must have a very large input impedance and gain, inevitably picking up noise due to corona discharge of the spark plug 18.
In order to solve this problem, a method for detecting knocking by extracting the knocking frequency components from the output of the ionic current detecting unit by using a band-pass filter, and integrating the knocking frequency components to reject noise has been already proposed (see Japanese Unexamined Patent Publication (Kokai) No. 6-159129).
FIG. 3 is a diagram illustrating the constitution of a device for detecting knocking. The output of the amplifying circuit 195 of the ionic current detecting unit 19 is supplied to an integrating unit 33 through an LC resonance masking unit 31 for masking LC resonance and a band-pass filtering unit 32. An integrated value of the ionic current which is the output of the integrating unit is supplied to the ignition timing control unit 14 and is used for detecting knocking and for controlling the ignition timing.
However, if the timing for resetting the LC resonance masking unit is overlapped by the timing in which the ionic current increases when the engine speed is high, the output of the LC resonance masking unit 31 may stepwisely change when the LC resonance masking unit is reset.
This stepwise change has a broad frequency range and causes, through the band-pass filtering unit 32, the output of the integrating unit 33 to be raised. Therefore, it is difficult to prevent this change from being erroneously determined as knocking.
FIG. 4 is a diagram illustrating the problems, and shows the output of the LC resonance masking unit 31, the output of the band-pass filtering unit 32 and the output of the integrating unit 33, and wherein the abscissa represents time.
That is, when the LC resonance masking unit is reset at t.sub.1, the signal input to the band-pass filtering unit 32 stepwisely changes from "0" to a given positive value. Therefore, noise due to this stepwise change appears on the output of the band-pass filtering unit 32. Accordingly, the output of the integrating unit 33 increases due to the noise and exceeds a knocking threshold value NL due to spike noise, generated at t.sub.2, erroneously discriminated as an occurrence of knocking.
The present invention was accomplished in view of the above-mentioned problems, and its object is to provide a device for detecting knocking in an internal combustion engine capable of preventing the accuracy for detecting knocking from being deteriorated by noise when the LC resonance masking unit is reset.